Electronic devices, such as electronic book readers (“eBook reader devices”), cellular telephones, portable media players, desktop computers, laptops, tablet computers, netbooks, personal digital assistants, and the like, rely on electrical power to function. Power sources provide electrical power to the electronic devices. Some power sources exhibit variable characteristics such as changing current and voltage output. For example, output voltage and current of a photovoltaic cell will vary as the level of light incident on the cell changes. These changing characteristics introduce difficulties in effectively transferring energy from the variable power source to a load within the electronic device.